Polycarbonate polymers are excellent molding materials as products made therefrom have high impact strength, toughness, high transparency, wide temperature limits (high impact resistance below -60.degree. C. and a UL thermal endurance rating of 115.degree. C. with impact), good dimensional stability, high creep resistance and electrical properties which qualify them as sole supports for current carrying parts.
Polycarbonates are, however, very difficult to fabricate from melts for the reason that the high temperatures required lead to thermal and oxidative degradation. This is especially aggravated in pigmented, e.g., titanium dioxide-containing, modifications. Degradation results in increases in melt viscosity and development of undesirable color. In the past, much effort has been expended in preparing thermally stable polycarbonate compositions which would be color stable at elevated temperatures and particularly at the high molding temperatures generally employed to prepare molded polycarbonate articles. Many different additives have been found that are quite suitable for rendering polycarbonates heat and color stable. Particularly useful are triorgano-phosphites which have been disclosed in U.S. Pat. No. 3,305,520. Also, U.S. Pat. No. 3,729,440 discloses a thermally stable aromatic polycarbonate containing a phosphonite and an epoxy compound. Further, U.S. Pat. No. 3,673,146 discloses a method for stabilizing polycarbonate resin involving using a triorganophosphite, cycloaliphatic epoxy compound, ultramarine pigment and aryl siloxane fluid. Although phosphites alone appear to have an advantageous stabilizing effect, in general, higher than desirable amounts are necessary. Furthermore, it is known from Caird, U.S. Pat. No. 3,087,908, assigned to the assignee herein, that polycarbonates can be admixed with polydiorganosiloxanes to produce melt processed materials which are free from blemishes, undesirable color effects, bubbles and craters. However, there is no suggestion that thermal oxidative stability can be improved by this expedient, especially when pigments are also present.
It has now been discovered that minor effective amounts of oligomeric or polymeric hydrocarbonoxysiloxanes are remarkably effective per se and in combination with organic phosphites and phosphonites and/or organic epoxide compounds to improve the thermal oxidative stability of aromatic polycarbonate resins. In contrast to the additives of Caird, incorporated herein by reference, the superiority of those herein is believed to be due to the unique presence of silicon bonded active hydrocarbonoxy groups--which confers anti-oxidant-thermal stabilizing properties in the additive.